Advanced version support for alias and params.

[FelicianoTech]

This PR completely reworks how cimg-shared handles version numbers on the command-line. The logic in this repository is handled by two scripts, release.sh and gen-dockerfiles.sh. Both have been updated to improve support, fix a bug, and lay the ground work for future features.

What in this PR:

1. gen-dockerfiles.sh now supports being passed multiple versions at once.
2. release.sh always supported multiple versions but it didn't work before num. 1 above. Now it has been adjusted to work in conjunction with gen-dockerfiles.sh.
3. release.sh now supports being passed an additional parameter the way gen-dockerfiles.sh has. Both now refer to this as param1 rather than sha. The more broad naming allows it to be used for more situations such as the download URL for the cimg-openjdk builds.
4. These scripts now have a concept of version groups. They can be passed just a version, or a version with a combination of an alias (for Docker tag purposes) or a generic parameter which can be anything that the Dockerfile templates might need. Versions can now also be prefixed with a v.
5. A bug fix. A recent PR in this repo enabled support for shared variants for images. This created a bug for images that doesn't currently have a variant (i.e. cimg-node) in which is was trying to process variants that weren't there. This PR fixes that.

What's Not in this PR:

1. Full alias support. The ground work for supporting aliases is in this PR, but the work to actually pass that info to Docker and eventually Docker Hub is not yet done. That will follow once this PR is merged.

Closes #15, closes #14, and closes #1.

[lokst]

LGTM